1. Field of the Invention
The invention relates generally to printed circuit boards (PCBs) and specifically to methods of producing copper clad epoxy PCBs suitable for use at the microwave frequencies processed in global positioning system (GPS) receivers.
2. Description of the Prior Art
Microwave circuit design has presented special challenges to circuit designers and manufacturing alike. At microwave frequencies, short wire leads appear to be significant inductors, stray capacitances become a major problem, and insulators and dielectrics can become lossy and become resistors. A phenomenon known as the "skin effect" causes currents in conductors to concentrate in the outer skin of the conductor more and more as the frequency increases. The vast majority of electronic circuit manufacturers have adopted the use of printed circuit boards (PCBs) as a quick, economical way to build circuits and systems. PCB construction is especially suited to high volume automated manufacturing, so devices using PCBs are inexpensive and affordable.
The most common materials used today are epoxy/fiberglass laminate boards with copper cladding on one or both sides. Photoresist is applied to the copper clad, and a pattern is used to expose the photoresist. A developer then hardens the exposed photoresist and the rest is washed away. A ferric chloride bath then is used to etch away the copper clad not protected by photoresist. The materials and techniques used make copper clad epoxy laminate PCBs a cost effective choice.
Ceramic and alumina substrates have been used almost exclusively as PCB substitutes at microwave frequencies because radio frequency (RF) losses are too great in the epoxy laminate. These substrates, however, are fragile and require expensive materials and manufacturing techniques to build electronic circuits. Multi-layer thick film alumina circuits are possible, but because the internal conductive layers use molybdenum, conductor resistances are a problem. Such construction is not an alternative when one wishes to build low-cost, personal GPS receivers that are within the economic reach of the majority of casual users.
Prior art copper clad epoxy laminates used in the manufacture of multilayer packages comprise single and double-sided copper clad epoxy laminates that have only single-treated copper foil. These treated sides are inside the epoxy substrate copper clad. The treating of the copper foil aids in the bonding of the copper foil to the epoxy substrate. Multilayer package fabricators then will use a deliberate step of oxidation to oxidize the outside of the single treated copper foil so that prepreg glass can be used to bond to another single-sided copper clad laminate. The result is a single-treated copper foil with a thick oxide layer that winds up in the middle of a multilayer package where it can reduce propagation speeds and increase insertion losses in devices operating at microwave frequencies.
What is needed is a double treated copper clad epoxy laminate PCB and a method for making multilayer packages, such as is described below in connection with the present invention.